Maguire's name was jeered when it was read out ahead of kickoff at the Melbourne Cricket Ground and his every touch was booed for roughly 30 minutes of the first half.
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Donny van de Beek, who came on as a half-time substitute, said afterwards he had heard the reception but backed Maguire and insisted the 29-year-old had played well.
"I heard as well," Van de Beek said. "I didn't know really what happened.
"But I think if I saw the game today I think Harry was playing really well. He was aggressive, he got so many balls, so that means he has a big personality.
"He has a lot of experience, so I think that's positive what he [showed], his performance today."
New United boss Erik ten Hag has insisted Maguire will remain as captain and will be considered a first-choice player next season despite competition from Raphael Varane, Victor Lindelof and new signing Lisandro Martinez.
"I think it's preseason and we are happy with the improvement we see but also you see a lot of work to do," Ten Hag said.
"We scored some wonderful goals and that is also true.
"We press all day and, if we can, do that high up the pitch. But if we can't, we get to do that in a lower block but we also have to press and do it together as a team.
"I see still switches we can avoid and we can prepare a better press. And I think, also on the ball, we play well but you see, in the end, when it's 2-0 or 3-0 up, the belief is increasing.
"And then, also on the ball, we can delay and we can postpone and we can fight opponents so they come out and we can speed up.
"The 2-0 [goal], especially, was a great goal."